Wireless communication method and system for minimizing interference by determining mobile station zone locations and potential conflicts between cell zones

ABSTRACT

A wireless communication method and system for minimizing severe interference to one or more wireless transmit/receive unit (WTRUs). A plurality of non-overlapping zones (e.g., an inner zone and an outer zone) is defined for each cell of a wireless multi-cell communication system, such as a time division duplex (TDD) system. Information regarding potential conflicts between the non-overlapping cell zones is obtained. Each potential conflict indicates that there is a high likelihood of one of the WTRUs using a specific time slot for transmitting in a zone of one cell causing severe interference to a second one of the WTRUs using the specific time slot for receiving in a zone of another (i.e., a different) cell. Time slot usage (e.g., uplink, downlink or none) is then determined on a zone-by-zone basis using the obtained information to minimize interference caused by a conflicting uplink assignment or a conflicting downlink assignment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/334,459 filed Dec. 30,2002, which issued as U.S. Pat. No. 7,046,655on May 16, 2006, which in turn claims priority from Provisionalapplication No. 60/403,685 filed Aug. 15, 2002, which are incorporatedby reference as if fully set forth.

BACKGROUND OF THE INVENTION

The present invention relates generally to radio communication systemsusing the time division duplexing (TDD) mode. More particularly, theinvention relates to assignment of slots and slot-to-cell assignments inradio communication systems using TDD.

Cellular systems generally divide the time axis into intervals of equaldurations called frames. Cellular systems employing the TDD schemedivide frames into a finite number of intervals of equal duration,called slots, and allow a cell to use some or all of the slots foruplink transmissions (mobile-to-base) or downlink (base-to-mobile)transmissions. The slot assignment of a cell defines how each slot isused by this cell. There are three possible ways for a cell to use aslot: 1) uplink transmissions; 2) downlink transmissions; or 3) the slotis not used.

The slot assignment of a cell can be varied by the system in order toadapt to the long-term variations of the traffic load. For example, thesystem may modify the assignment of one slot from uplink to downlink ifthe intensity of downlink traffic increases while the uplink trafficdecreases. In addition, different cells of a system do not generallyneed to have the same slot assignment. Accordingly, if trafficcharacteristics in one geographical area are different from anotherarea, the cells covering those areas may have different slot assignmentsto best adapt to local traffic conditions.

In the prior art, a simple approach in order to avoid base-to-base andmobile-to-mobile interference is to use the same slot assignment for allcells in the same geographic area; only allowing different assignmentsbetween cells that are clearly isolated from each other. The obviousdisadvantage of this approach is when cells are deployed in a way toprovide continuous coverage, as is often the case, it is difficult tosegregate one subset of cells from another, unless the use of certainslots are completely disallowed in some cells. This ultimately resultsin a capacity loss of the system.

Therefore, both mobile-to-mobile and base-to-base interference restrictthe use of independent slot assignments between cells in the samegeographical area. What is needed is a system which avoids the loss ofcapacity as the traffic asymmetry metric varies over a coverage area.

SUMMARY

The present invention is related to a wireless communication method andsystem for minimizing severe interference to one or more wirelesstransmit/receive unit (WTRUs). A plurality of non-overlapping zones aredefined for each cell of a wireless multi-cell communication system,such as a TDD system. Information regarding potential conflicts betweenthe non-overlapping cell zones is obtained. Each potential conflictindicates that there is a high likelihood of one of the WTRUs using aspecific time slot for transmitting in a zone of one cell causing severeinterference to a second one of the WTRUs using the specific time slotfor receiving in a zone of another (i.e., a different) cell. Time slotusage (e.g., uplink, downlink or none) is then determined on azone-by-zone basis using the obtained information to minimizeinterference caused by a conflicting uplink assignment or a conflictingdownlink assignment. In one embodiment, the location of a WTRU istracked by determining whether the WTRU is located in an inner or outerzone of a particular cell.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred embodiment, given by way of exampleand to be understood in conjunction with the accompanying drawingswherein:

FIG. 1 shows an example of division of cells into zones;

FIG. 2 illustrates how a cell can be segmented into two simple zones;

FIG. 3 is a look-up table showing zones that are conflicting with eachother; and

FIG. 4 illustrates the border of two neighboring cells and a WTRU inclose proximity to the border.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to the drawingfigures wherein like numerals represent like elements throughout.

Although the present invention will be described with reference to theexample shown in FIG. 1 for the case of a hexagonal deployment, thepresent invention is readily extensible to other types of deployment. Itis up to the system administrators and operators to define the zonesaccording to their specific situations. The cell division shown in FIG.1 is only an example of one type of cell division, it should be notedthat in an actual deployment that there are other ways to divide cellsinto zones.

As illustrated in FIG. 1, cell A 100 has been divided into six (6) zonesA1-A6 and likewise, the neighboring cells B 110 and C 120 have each beendivided into six (6) zones B1-B6 and C1-C6, respectively. Several WTRUs130, 140 are also randomly located. For the first embodiment it isassumed that the WTRUs are not equipped with adaptive antennas. Thefirst WTRU 130 is located in sector A6 and a second WTRU 140 is locatedin sector B3.

The method and system of the present invention permit two neighboringcells to use conflicting slot assignments (i.e., a slot is used for theuplink in one cell but for the downlink in the neighboring cell) basedon the concept of “zones.” A zone is a subdivision of a cell defined bythe system operator. Cells are divided in a certain number of(non-overlapping) zones. Two zones (belonging to different cells) aredefined to be conflicting with each other if there is a high likelihoodthat a first WTRU transmitting in one zone would cause severeinterference to a second WTRU receiving in the second zone. Thedetermination of whether two zones are conflicting with each other canbe made by analysis of the cell layout. Alternatively, one couldenvision a more sophisticated scheme based on collecting measurementsmade by WTRUs.

Although the method of determining conflicting zones will be describedwith reference to FIG. 3, this embodiment is illustrative and should notbe construed as the only procedure for determining such conflictingzones. The conflicting zone look-up table in FIG. 3 facilitatesdetermining the conflicting zones. For example, the first WTRU 130 islocated in zone A6. Zone A6 is first located in the vertical column ofthe conflict lookup table. An X in its corresponding row indicates thatzone B3 is a potential conflicting zone and a “

” in the table indicates an entry corresponding to a zone of the samecell (e.g. A1, A2, A3, A4 and A5). Because a slot cannot be used forboth uplink and downlink simultaneously in the same cell, zonesbelonging to the same cell cannot be allowed to have opposingdirections. This means that the transmissions from a first WTRU 130 inzone A6 have the potential to cause interference to a second WTRU 140 inzone B3. Interference would occur if both WTRUs 130 and 140 use the sameslot for transmission and reception, respectively.

Having determined the conflicting zones, the physical resources can nowbe used more flexibly and efficiently. For example, suppose that it isdesired that a given slot (S) be used for the uplink in cell A and inthe downlink in cell B. Such a situation can happen if, for example,cell B has more downlink traffic than cell A. Without using the conceptof zones, it would be difficult for cell A and cell B to use slot S indifferent directions. This is because a WTRU of cell A transmitting inslot S near the border of cell B would create too much interference toWTRUs receiving in slot S in cell B. However, when the concept of zonesis used this problem is surmounted in the following way. Slot S can beused by some WTRUs of both cells A and B, provided that it is not usedsimultaneously by two WTRUs in conflicting zones. For example, supposethat zone A3 on FIG. 1 is not conflicting with any zone of cell B.Suppose also that slot S is used for the uplink in cell A and for thedownlink in cell B. A WTRU in zone A3 would be allowed to use slot S(for the uplink) even though that slot may be used for the downlink incell B. This represents a flexibility advantage and ultimately acapacity advantage, since otherwise slot S would have been completelyunavailable for uplink in cell A.

In the description of FIG. 3, it is assumed that the location of theWTRU is known. However, as the WTRU traverses through the coverage area,the system must keep track of the zone it is currently located in. Forexample, if the WTRU starts out in zone C3 and travels across thecoverage area to zone B4, the WTRU would travel through zones C3, C4, B3and B4 on its route. Based on the knowledge of the lookup table showingconflicting zones, the system can determine the best use of each slot(uplink, downlink or none) in every zone, taking into account theconstraints imposed by the conflicts between zones and the averagetraffic characteristics. This is known as the slow dynamic channelallocation (SDCA) process, which occurs over a relatively large timescale (hours, days, weeks or months). The SDCA uses the conflict zonelookup table in this determination of slot usage.

Given the framework of slot usage defined by the SDCA process, when aWTRU connects to the system it is allocated at least one downlinkchannel in one or more of the slots which can be used for the downlinkin the zone where it is located. It is also allocated at least oneuplink channel in one or more of the slots that can be used for theuplink. Whenever a WTRU moves to another zone, the system checks if thechannel allocation needs to be changed, a problem situation which canoccur if the slot(s) used by this WTRU are no longer allowed in a givendirection in the new zone it is moving in. This can be achieved by thefast dynamic channel allocation (FDCA) process. Therefore, there is noneed to re-consult the conflicting zone lookup table every time a WTRUmoves to a new zone. Rather, each zone is associated with a possibleusage, uplink, downlink or none for every slot which is determined bythe slow DCA based on the lookup table. The system uses this slot usageinformation when it is time to assign channels to a WTRU. The systemthen makes the appropriate non-interfering uplink and downlink slotassignments.

Another advantage of the present invention is the use of slot allocationto affect load balancing. Referring to FIG. 4, if the traffic loads ineach of two adjoining cells 300, 310 have differing asymmetry usecharacteristics, (that is, the first cell 300 is mostly downlinktraffic, while the second cell 310 is mostly uplink traffic), withoutconflict zone management, the transmissions of WTRU 32 would causeinterference with the first cell 310, if the cells are using the sameslot for different directions. A fast allocation protocol could attemptemploy an escape mechanism, which is a means for a channel allocation ofa specific WTRU to be changed due to excessive interference as detectedby a particular WTRU using this channel. The use of escape mechanisms isunsatisfactory since it would result in effectively forbidding the useof that slot in significant parts of the cell. Conflict zoning allows amobile to traverse through a coverage area, (including zones), whilehaving negligible impact on the cell's usage characteristics.

In an alternative embodiment in accordance with the present invention, asimple zone division scheme is used. A cell is separated into two zonesas shown in FIG. 2. The first zone is the outer zone 21, which isdefined by the coverage area 24 of the cell, (exclusive of the innerzone 22); and the second zone is the inner zone 22. The serving basestation 20 is located at the center of the inner zone 22.

In one embodiment, the system determines both the cell in which the WTRU28 is located and whether the WTRU is in an inner zone 22 or the outerzone 21. First measurements of the signal delay and the received signalpower are performed. This allows the determination of whether WTRU 28 islocated in the inner or outer zone. The example in FIG. 2, illustrates aWTRU 28 located in the inner zone 22, and the WTRU 26 is located in theouter zone 21.

Another method to determine location utilizes additional nearby basestations or other WTRU. However, because a WTRU requires constanttracking as it traverses the coverage area, there is a need forcontinuous system participation and coordination of the other basestations and WTRUs causing this to be a very resource intensivetechnique

An additional technique to determine the location utilizes globalpositioning satellites of a Global Positioning System (GPS). A GPSreceiver is put in each WTRU to identify the location of the WTRU. Thecoordinates are reported by the WTRU to the base station and the systemuses the coordinates as aforementioned. However, there are somedisadvantages with this option. The first is the need for the WTRU to bein a favorable position allowing it to properly receive the satellitesignals (outdoors). Also, since the measurement is performed by theWTRU, the WTRU needs to constantly transmit location information to thesystem, which increases the signaling burden over the air interface andutilizes precious battery resources.

The consistency and accuracy of zone location is improved with adaptiveantennas. The location measurements including inclination angle andsignal level readings are employed to determine the position the WTRU.An advantage to adaptive antennas is that a position is obtained withoutthe need of measurements from any other base stations or WTRUs.Therefore, adaptive antennas provide an efficient and independent meansfor tracking WTRUs.

The cellular system has two types of interference to cope with, thefirst is base-to-base interference and can occur when a first basestation's downlink is another base station's uplink and the uplink basestation receives the other base station's downlink, which impedes ordegrades the intended uplink signal. The second type of interferencethat can occur in the cellular system (mobile-to-mobile interference)occurs when a first mobile's reception is impeded or degraded by anothermobile's transmission.

Adaptive antennas can be placed at the base station, on the WTRUs or atboth the base station and the WTRUs. The performance of the zonedivision scheme depends on basically two factors: 1) the effectiveness(i.e. reliability and convenience) with which one can determine the zonewhere a user is situated and the ability to track a moving user; 2) thesize of the neighborhoods of the zones. The neighborhood of zone Z isdefined as the set of other zones in other cells that conflict with zoneZ. This means that the entries corresponding to Z and any zone belongingto the neighborhood of Z should be checked in the lookup table of FIG.3.

The use of zones for slot allocation requires locating the WTRU withreasonable certainty. As the accuracy of the WTRU positioning isincreased, it is possible to define a larger number of zones per cell,resulting in higher flexibility and increased efficiency of the system.

A small neighborhood is one defined as one with a small number of zones.This means that for every zone, there is a relatively small number ofother zones that are conflicting with it. Thus, there are fewerconstraints, which allow for more flexibility in the determination ofthe slot usage for every zone. This allows for the greatest flexibilityin terms of assigning different proportions of uplink versus downlinktraffic for different cells. Therefore, any scheme that tends torestrict the size of neighborhoods results in a gain of flexibility.

When two zones are conflicting with each other, this may be due to oneor both of the following:

-   -   a) The probability of base-to-base interference is high if the        zones are using slots in opposite directions    -   b) The probability of mobile-to-mobile interference is high if        the zones are using slots in opposite directions.        When the mobiles are also equipped with adaptive antennas there        will be less pairs of zones that are conflicting with each other        because the overall probability of mobile-to-mobile interference        is decreased. This is because mobiles equipped adaptive antennas        tend to transmit and receive energy from specific directions,        which reduces the probability that one mobile interferes with        another. To say that there will be less pairs of zones that are        conflicting with each other is the same as saying that the sizes        of the neighborhoods of every zone is reduced under the        definition of neighborhood of a zone.

The size of neighborhood depends of several factors, such as thespecific geography of deployment or the propagation conditions present.If the mobiles are equipped with adaptive antennas, the neighborhood ofa zone may be restricted to fewer zones. In the best case scenarios, aneighborhood can even be limited to only one or two zones, if thedeployment is such that there are few scatterers around the mobiles.This is because the probability of mobile-to-mobile interferencediminishes when mobiles are transmitting using narrow beams. Similarly,when adaptive antennas are used at the base station, the sizes of theneighborhoods should be reduced for the same reason.

In another embodiment the transmission power of mobiles is taken intoaccount. The transmitted power is regarded as an important factor andinfluences the size of a neighborhood. If a zone Z is geographicallydefined as being close to the base station, any WTRUs transmitting inthat zone will tend to transmit at a lower power level since they arecloser to the base station and will need less power than other WTRUsthat are further away. They per se generate less interference than otherWTRUs and are less susceptible to the affect of other mobiles that wouldbe receiving in the same timeslot. As a result, the number of zones thatare conflicting with this zone Z tends to be smaller. In other words, aneighborhood of a zone close to the serving base station will normallybe smaller than the neighborhood of a zone closer to the edge of thecell.

As stated above, mobiles normally transmit at a lower power when theyare closer to their serving base station. But, with the advent of newhigh data rate cellular technology, an increase in power is oftennecessary to facilitate high data rate exchange at both the mobile andthe base station. To accommodate the higher data rates, the WTRUs andthe serving base station will have to increase transmission power, evenif the WTRU is close to the serving base station. Therefore, it may benecessary to define service-dependent neighborhoods.

For example, it may be possible that two zones which are not conflictingwith each other if the WTRUs are using the voice service or a low-rateservice, but they would be conflicting if the WTRUs were using ahigh-rate service, for example a data rate of 384 kbps. In that case, aWTRU could be allowed to use a certain slot in a given zone only if itis using a low-rate service.

Although the present invention has been described in detail, it is to beunderstood that the invention is not limited thereto, and that variouschanges can be made therein without departing from the spirit and scopeof the invention, which is defined by the attached claims.

1. A method of minimizing interference in a wireless multi-cellcommunication system that serves a plurality of wirelesstransmit/receive units (WTRUs), the method comprising: (a) defining aplurality of non-overlapping zones in a conflicting zone lookup tablefor each cell in the system; and (b) determining time slot usage on azone-by-zone basis using information obtained from the conflicting zonelookup table to minimize interference caused by a conflicting uplinkassignment or a conflicting downlink assignment.
 2. The method of claim1 wherein the information obtained from the conflicting zone lookuptable indicates potential conflicts between the non-overlapping cellzones, the conflicting zone lookup table specifically identifies each ofthe zones of the cells, and each potential conflict indicates that thereis a high likelihood of one of the WTRUs using a specific time slot fortransmitting in one of the specifically identified zones of one cellcausing severe interference to a second one of the WTRUs using thespecific time slot for receiving in one of the specifically identifiedzones of another cell.
 3. The method of claim 2 wherein the wirelessmulti-cell communication system assigns channels to the WTRUs.
 4. Themethod of claim 2 wherein a current zone location is determined for eachof the WTRUs based on a signal delay measurement.
 5. The method of claim4 wherein the determination of the current zone location for each of theWTRUs is further based on a received power measurement.
 6. The method ofclaim 4 wherein the current zone location identifies a specific cell ofthe system in which the WTRU is located.
 7. The method of claim 4wherein the non-overlapping zones include an inner zone and an outerzone, and the current zone location indicates whether the WTRU islocated in the inner zone or the outer zone.
 8. The method of claim 2wherein the wireless multi-cell communication system is a time divisionduplex (TDD) system.
 9. The method of claim 2 wherein the specific timeslot can be simultaneously used by WTRUs in zones of different cells ifthe conflicting zone lookup table indicates that the different cellzones do not conflict.
 10. The method of claim 2 wherein a slow dynamicchannel allocation (SDCA) process is used to determine the best use ofeach time slot in each cell zone based on constraints imposed byconflicting zones indicated by the conflicting zone lookup table, andaverage traffic characteristics.
 11. A wireless multi-cell communicationsystem comprising: (a) a plurality of wireless transmit/receive units(WTRUs); (b) a plurality of cells, each of the cells having a pluralityof non-overlapping zones; (c) means for defining a plurality ofnon-overlapping zones in a conflicting zone lookup table for of thecells in the system; and (d) means for determining time slot usage on azone-by-zone basis using information obtained from the conflicting zonelookup table to minimize interference caused by a conflicting uplinkassignment or a conflicting downlink assignment.
 12. The system of claim11 wherein the information obtained from the conflicting zone lookuptable indicates potential conflicts between the non-overlapping cellzones, the conflicting zone lookup table specifically identifies each ofthe zones of the cells, and each potential conflict indicates that thereis a high likelihood of one of the WTRUs using a specific time slot fortransmitting in one of the specifically identified zones of one cellcausing severe interference to a second one of the WTRUs using thespecific time slot for receiving in one of the specifically identifiedzones of another cell.
 13. The system of claim 12 wherein the wirelessmulti-cell communication system assigns channels to the WTRUs.
 14. Thesystem of claim 12 wherein a current zone location is determined foreach of the WTRUs based on a signal delay measurement.
 15. The system ofclaim 14 wherein the determination of the current zone location for eachof the WTRUs is further based on a received power measurement.
 16. Thesystem of claim 14 wherein the current zone location identifies aspecific cell of the system in which the WTRU is located.
 17. The systemof claim 14 wherein the non-overlapping zones include an inner zone andan outer zone, and the current zone location indicates whether the WTRUis located in the inner zone or the outer zone.
 18. The system of claim12 wherein the wireless multi-cell communication system is a timedivision duplex (TDD) system.
 19. The system of claim 12 wherein thespecific time slot can be simultaneously used by WTRUs in zones ofdifferent cells if the conflicting zone lookup table indicates that thedifferent cell zones do not conflict.
 20. The system of claim 12 whereina slow dynamic channel allocation (SDCA) process is used to determinethe best use of each time slot in each cell zone based on constraintsimposed by conflicting zones indicated by the conflicting zone lookuptable, and average traffic characteristics.